July 29, 2019

The Driest Day of the Year in Seattle, But What About Other Places?

Today is a very special day here is Seattle:  climatologically the driest day of the year.   Particularly nice because our sky is smoke free and temperatures will be temperate.   The dry nature of July 29th, is shown by a plot of the historical frequency of receiving .01 inches of precipitation at Seattle-Tacoma Airport, whose record stretches back to the last 1940s (see below).  Only about 8% of July 29ths, have received rain.  By the end of August, the climatological probabilities jump to 25%!

This is why I recommend that folks plan their big events (outdoor weddings, family barbecues, etc.) on or near this date.  So if you were planning on getting married, you better call your relatives and friends right now.

Why so dry?    Major weather systems are shunted to the north in late July and summer thunderstorms  are rare over western Washington because of the cool Pacific Ocean.

But what about other locations in the Northwest?  Is July 29th their driest day?   Let's check!

Spokane's Airport?  July 29th is quite dry, but not the driest, with virtually an equal chance of super-dry conditions (about 10% chance) from mid-July until early August.  Isolated from ocean drizzle and protected by the Cascades, their precipitation doesn't come back as quickly as in Seattle.   And of course, there is a bit of randomness to the exact driest day, depending on whether an errant shower hits a location.

What about the coast at Hoquiam?  Generally similar to Seattle, but the driest day is a few days earlier (July 25th).  And the probability of light rain comes up faster (about 30% by Sept 1).

But just when you think that all regional locations have the same driest periods, we check out Yakima, which is in the very dry lee of the Cascade mountains (see below).  July and August are all dry, but the two driest days are in early and mid-July.  These are days in which it HAS NEVER RAINED.   Yakima is a different story because they get little precipitation from incoming weather systems, with most of their summer rain from thunderstorms, which have a more random component and forced in different ways (e.g.. approaching upper trough, monsoon moisture coming up from the southwest).

Finally, to throw a few bones to readers in Oregon, here is Portland.  They get down to 6%, with July 29th not being lowest.  Their dry period is longer than Seattle, roughly from early July to early August.  Probably because they are slightly further south, with a small reduction of precipitation from weather systems passing to the north.

Eugene, Oregon is even drier than Portland, with several days in July never experiencing rain and an uber-dry period from roughly July 10th through late August.    

So enjoy our dry period and this year we are blessed with minimal smoke over the region---and I suspect that will hold for a while.  There are very few BC and California fires compared to last year and the mainly grass fires started by lighting in eastern WA will be rapidly contained.


AnnouncementL  I will be giving a talk "The Great Storms of the Pacific Coast" in Ocean Shores at 6:30 PM on September 7th at the Shilo Inn as part of the Coastal Interpretative Center's summer lecture series.  More information is found here: https://www.interpretivecenter.org/.   Shilo Inn is offering special room  rates for those wishing to stay overnight, as well as a special buffet.   


  1. Significant date for Terrace, located in northwestcentral British Columbia, Canada as well. Today we celebrate the 10 year anniversary of the all time record high temp set for the town; a scorching 103 degrees F in the downtown concrete jungle. Juxtaposed with today's conditions: A rare cool night (great for sleep!) where overnight lows dipped to the single digits on the C scale and forecasted high temps near half of what the record breaking day brought ten years ago.

    1. I was in Bella Coola last week and it rained alot! One day was heavy rain it felt like October. Beautiful place though I couldn't believe the scenery it's like Norway or New Zealand.

  2. Anyone know where those graphs come from? I'm curious as to what it is for my city which is further North of Seattle. :)

    1. These are from NOAA's Western Regional Climate Center, which has a huge amount of information including various tools to generate graphs and reports.


      Some of the WRCC's website are to links outside of WRCC as well.

  3. 8% chance might seem like zero to some, but it is not. In this driest week of the year, on any given day, the chances of measurable precip are in the 10% neighborhood.

    I recall that I organized an outdoor annual meeting in late July for a company that I worked for in Kent and we had about a quarter inch of rain. I believe the year was 1993. Luckily, we had a large tent (I had incorrectly assumed it was needed more to block the sun, but on this day, that was not the case.)

  4. It is odd that there is not more fine structure to the peaks. For example, regarding Seattle, compare Feb 13 to Feb 18 (I am estimating; the resolution is so-so). And some of the anomalies are surprisingly broad. Why so much difference? I can only imagine that simply reflects the fact that we have not been collecting data for all that long.

    Also, again regarding Seattle, it is notable that a smoothed curve would appear almost flat in May.


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